Series collinear antenna segments are well known in the field of antenna design. They have a number of advantages over other collinear antenna strategies such as a corporate feed collinear because of their ease of construction and associated affordability. They consist of a number of alternate radiating elements and inter-element phasing sections resulting in a phased array antenna.
Each radiating element is optimally fed in phase so that each of the radiating elements will radiate in unison. This enables the focussing of the antenna radiation pattern. Each individual radiating element is designed to be of a specific physical length in order to provide the most effective radiation of power for a given wavelength. Following each active radiating element is an inter-element phasing section, wherein the radiation from the antenna is suppressed until the next correct phase point on the wavefront is reached, wherein another radiating element is fed in series.
The ideal theoretical inter-element phasing section would see the suppression of ½ λ (180 degrees of phase) of the wave front, where λ is the design wavelength for the antenna. Also ideally, the physical length of the radiating element should be ½ λ. In addition, the ideal theoretical physical spacing between the two radiating elements would be ¼ λ as measured from the top of one radiating element to the bottom of the next radiating element. Clearly, there are competing design constraints here which make realisation of the theoretical ideal difficult. Furthermore, in practice, users require coverage over a range of wavelengths. When the wavelength in use changes from the design wavelength, the side lobes of the antenna radiation pattern become more pronounced. Also elevation tilt in the radiation pattern is induced when the individual radiating elements are not fed precisely in phase with each other.
There a number of approaches in the prior art which attempt to realise this theoretical ideal. The most common is the Franklin collinear array. Most such Franklin antennas are manufactured using a coaxial cable feed line, and the velocity of propagation, vp, of the coaxial cable can help the designer get closer to the theoretical ideal. By making use of a reduced vp in the inter-element phasing section, the physical length associated with a ½ λ phase difference can be reduced somewhat. However, this approach is a compromise and as more radiating elements are added to the series collinear antenna segment the errors introduced become compounded.
Another approach is to use a ½ λ wire phasing coil for the inter-element phasing section. Coil based series collinear antenna segments such as this have ½ λ phase elements which are separated by the ideal physical spacing of ¼ λ. However, although these coils include both inductive and capacitive components, their capacitance is high and thus the Q factor and hence the wavelength sensitivity is high. This implies that the introduced phase difference may well be 180° at the design wavelength, but then vary significantly with wavelength in comparison with a coaxial inter-element phasing section as adopted in the Franklin approach. Therefore these designs are essentially narrowband. They are not used where extended bandwidths are required due to the performance degradation in pattern stability which results from the variation in the phase difference with wavelength. Another significant disadvantage is that the physical structure of the coils must be very tightly controlled, especially when designing for short wavelengths thus adding to the cost of manufacture. Moreover, the coils themselves must be made of a material which is sturdy enough to support itself physically.
A natural extension to this coil design approach is to replace the physical coils with one which is reproduced entirely on a circuit board. Such attempts have included a helical coil which is printed on the outside of a round former, simply reproducing the physical coil. This approach effectively simulates a physical coil but it is also expensive and has not seen acceptance. In addition this approach also fails to address the large variation in phase introduced as a function of wavelength.
Another further approach to approximate a coil is to implement a meander on a flat circuit board. This does provide a high inductance, lower capacitance inter-element phasing section due to the low capacitance of the tracks on the circuit board but consequently the matching ability of a flat meander is significantly degraded. This is because the radiating elements and the flat meander are not well de-coupled from each other and hence the definition between these two components of a series collinear antenna segment is poor, resulting in reduced bandwidth and performance. Consequently this approach is used only for smaller, lower gain antennas, where performance is not critical. Other electrical components can be added to series collinear antenna segments which use a flat meander inter-element phasing section to introduce the desired capacitance. However, this results in significantly increased costs of production.
Accordingly it is an object of the present invention to provide for series collinear antenna segments, and antennas, with improved broad band characteristics.
It is a further object of the invention to provide for series collinear antenna segments, and antennas, with improved broad band characteristics and which are convenient and low cost to manufacture when compared with prior art designs.